In the search for high performance materials, considerable interest has been focused upon carbon fibers. The terms "carbon" fibers or "carbonaceous" fibers are used herein in the generic sense and include graphite fibers as well as amorphous carbon fibers. Graphite fibers are defined herein as fibers which consist essentially of carbon and have a predominant x-ray diffraction pattern characteristic of graphite. Amorphous carbon fibers, on the other hand are defined as fibers in which the bulk of the fiber weight can be attributed to carbon and which exhibit an essentially amorphous x-ray diffraction pattern. Graphite fibers generally have a higher Young's modulus than do amorphous carbon fibers and in addition are more highly electrically and thermally conductive. It will be understood however, that all carbon fibers including amorphous carbon fibers tend to include at least some crystalline graphite.
Industrial high performance materials of the future are projected to make substantial utilization of fiber reinforced composites, and carbon fibers theoretically have among the best properties of any fiber for use as high strength reinforcement. Among these desirable properties are corrosion and high temperature resistance, low density, high tensile strength and high modulus. During such service, the carbon fibers commonly are positioned within the continuous phase of a resinous matrix (e.g. a solid cured epoxy resin). Uses for carbon fiber reinforced composites include aerospace structural components, rocket motor casings, deep-submergence vessels, ablative materials for heat shields on re-entry vehicles, strong lightweight sports equipment, etc.
As is well known in the art, numerous processes have heretofore been proposed for the thermal conversion of organic polymeric fibrous materials (e.g. an acrylic multifilamentary tow) to a carbonaceous form while retaining the original fibrous configuration substantially intact. See for instance, the following commonly assigned U.S. Pat. Nos. 3,539,295; 3,656,904; 3,723,157; 3,723,605; 3,775,520; 3,818,082; 3,844,822; 3,900,556; 3,914,393; 3,925,524; 3,954,950; and 4,020,273. During commonly practiced carbon fiber formation techniques a multifilamentary tow of substantially parallel or columnized carbon fibers is formed with the individual "rod-like" fibers lying in a closely disposed side-by-side relationship.
In order for the resulting carbon fibers to serve well as fibrous reinforcement within a continuous phase of resinous material it is essential that the individual fibers be well dispersed within the matrix-forming resinous material prior to its solidification. Accordingly, it is essential when forming a composite article of optimum physical properties that the resinous material well impregnate the multifilamentary array of the carbon fibers so that resinous material is present to at least some degree in interstices between the individual fibers. If this does not occur resin rich areas will tend to be present in the resulting composite article. See, for instance, the disclosures of U.S. Pat. Nos. 3,704,485; 3,795,944; 3,798,095; and 3,873,389 where the pneumatic spreading of such carbon fibers was proposed prior to their resin impregnation. It has been found, however, that the pneumatic treatment of the fibers to accomplish decolumnization without spreading has tended to damage and to weaken to an excessive degree the relatively delicate fibers frequently to the extent of fiber breakage thereby creating an additional problem for those who choose to practice this additional process step and/or those carrying out the subsequent processing of the fibrous material.
It is an object of the present invention to provide an improved process for the production of a carbon fiber multifilamentary tow which is particularly suited for resin impregnation beginning with an acrylic fibrous precursor.
It is an object of the present invention to provide an improved process which may be carried out on a reliable and predictable basis for the production of a carbon fiber multifilamentary tow which is particularly suited for resin impregnation.
It is an object of the present invention to provide an improved process for the production of carbon fiber multifilamentary tow wherein the substantially parallel relationship of the individual filaments is disrupted in the substantial absence of filament breakage with the filaments becoming at least partially decolumnized.
It is an object of the present invention to provide an improved process for the production of carbon fibers which may be incorporated in a resin matrix to form a quality substantially void-free composite article which performs well in core crush and compression beam testing.
It is an object of the present invention to provide a multifilamentary tow and carbonaceous fibrous material containing at least 70 percent carbon by weight wherein the filaments thereof are substantially decolumnized and are capable of being readily impregnated by and dispersed within a matrix-forming resin.
It is an object of the present invention to provide a multifilamentary tow of carbonaceous fibrous material containing at least 70 percent carbon by weight wherein the filaments present therein are substantially decolumnized, which handles well, may be readily woven, and which is substantially free of deleterious surface fuzz.
It is a further object of the present invention to provide an improved process for forming an at least partially decolumnized carbon fiber multifilamentary tow which does not require the need for pneumatic filament spreading and the expense associated with the compression and supply of the required compressed air.
These and other objects, as well as the scope, nature, and utilization of the claimed process will be apparent to those skilled in the art from the following detailed description and appended claims.